In recent years, we have seen efforts to realize a network system (hereinafter referred to as “sensor net”) for bringing in various information of the actual world in real time into an information processing device by adding a small electronic circuit having a radio communication function to the sensor. A wide range of application is considered for the sensor net. For example, a proposal for its medical application is the use of a small electronic circuit integrating a radio communication circuit, a processor, a sensor and a battery to constantly monitor biological information such as pulse and the like. The monitor result is transmitted to a diagnosis device by radio communication and it is used to judge the health condition of the user.
In order to promote extensively the actual use of the sensor net, it is important to make the electronic circuit having a radio communication function, a sensor and a power source such as battery (hereinafter referred to as “sensor node”) maintenance-free over a long period of time and capable of transmitting continuously sensor data, and to miniaturize its external dimension. For this reason, efforts have been made to develop a miniature sensor node that can be placed anywhere.
In such a sensor node, in order to reduce the power consumption, it has been considered to operate intermittently the sensor, the CPU and the like to restrict the consumption of the battery. As a sensor node having a sensor and a CPU for sensing intermittently, for example the one described in JP-A No. 2005-3229856 is known. In this example, a sensor for collecting the information of a tire state detecting sensor and the like operating intermittently and for transmitting the same by radio communication is disclosed. Based on signals emitted from a constantly operating low-current oscillator, the CPU is started intermittently to supply the sensor and the radio communication unit with power, to enable the sensor to observe and transmit the observation values. And when the transmission of the sensing data is completed, the power supply to the CPU, the sensor and the radio communication unit is cut off.
As a system for cutting down the power consumption of the CPU controlling the sensor, the one that keeps the sensor constantly in operation and switches the CPU from the standby state to the operational state when any sensing data have developed (for example, JP-A No. 2001-59752) is known.